Theorem: dS/db + D S = 0 for S(b) = S_0 e^{-b D} (DWI signal-decay ODE)

Theorem (Stejskal-Tanner-ODE canonical): for S(b) = S_0 exp(-b D), dS/db = -D S_0 exp(-b D) = -D S, so dS/db + D S = 0 identically. Canonical sympy pins: S0, b, D = sp.symbols('S0 b D', positive=True); S = S0 * sp.exp(-b*D); residual =…